The present invention relates generally to a packaging machine, and more particularly to a packaging machine in which articles to be packaged are wrapped in heat-shrinkable plastic film. More specifically, the present invention relates to a system and a method for controlling the film cutting and sealing operation of such a machine.
Packaging machines which wrap articles in heat-shrinkable plastic film are well-known. One example of a machine of this general type can be seen in U.S. Pat. No. 4,658,569. A web of film is provided in which the film is folded to form a two-ply web having a fold connecting the layers along one edge of the web. The web is drawn from the roll, and turned through 90.degree. to form an open cavity within the web into which the article to be packaged is introduced. The film and article are together advanced beyond a sealing station. At the sealing station, a cooperating cutting/sealing head and anvil are positioned for vertical movement to bring the sealing/cutting head into contact with the anvil and to subsequently separate them. The head is heated, and the contact is made with the web located between the head and anvil. This causes the web to be both severed and sealed at the sealing station. Vertical movement of the sealing apparatus usually encompasses movement of both the sealing/cutting head and the anvil toward and away from each other, the movement typically being caused by one or more pneumatic cylinders.
Other machine operations seal the film along the unfolded side edge of the web, producing an article which is encased in the film. The sealed package is then moved into a heated shrink tunnel which causes the film to shrink so as to closely conform to the packaged article.
With regard to the lateral seal formed at the sealing station, it has generally been found that the seal should be positioned at the mid-point of the article height so that the upper and lower film layers are fitted around the article in a generally symmetrical manner. In one known machine of this type, this can be accomplished by moving the entire sealing head/anvil/pneumatic cylinder assembly up or down to properly position the meeting point between the sealing/cutting head and anvil which continue to be moved the full stroke of the cylinders.
It is desirable to minimize the time required for the sealing operation in order to enhance the overall speed of the process, especially when long continuous runs are made. One way to achieve this is to reduce the time required for the sealing head and anvil to complete their downward/upward travel for each cycle. This can be done, for packages which are of less than maximum acceptable height, by mechanically holding the anvil at its position below the plane of the conveyor system, while lowering the cutting/sealing head. This reduces the up/down stroke distance for each cycle, and also satisfies the desire to position the resulting seal along the mid-point of the package height.
However, this in turn creates a problem because the seal head speed and cushioning cannot be controlled by conventional methods. In conventional machines in which the seal head executes its full stroke for all package heights, the speed of the seal head is slowed (cushioned) just prior to contact with the anvil to provide smooth yet firm sealing pressure. This is accomplished through the conventional construction of the pneumatic cylinder itself, which pneumatically slows cylinder piston movement near the full limit of either direction of stroke. However, this method is not usable for less than the full stroke of the cylinder.
Slowing of the cutting/sealing head and anvil just prior to meeting is desirable. Otherwise, the full-speed "crashing" of the cutting/sealing head into the anvil can cause premature wearing of the cutting knife, as well as poor sealing quality due to "bounce" of the sealing mechanisms. Since many articles to be packaged on a machine can be expected to be less than the maximum article size accepted by the machine, a need exists for a control system for the cutting and sealing operation that permits high speed operation of the sealing station even when the articles to be packaged are of less than full height.